This invention relates to magnetotelluric geophysical exploration and more particularly to recording magnetotelluric signals along lines of exploration on the surface of the earth.
It has long been known that telluric currents circulate beneath the surface of the earth. The prior art suggests that the measurement of these naturally occuring currents will indicate the resistivity of the earth and hence the geophysical characteristics of the subsurface. These telluric currents are subject to variations caused by external influences, such as sunspot activity. Because these variations are unpredictable, measurement of telluric currents was little used as a geophysical exploration tool until Louis Cagniard devised a mathematical technique capable of accounting for these variations. His work is represented, for example, in U.S. Pat. No. 2,677,801 wherein the proposes measuring not only the telluric currents, but also the magnetic field at a measurement station on the earth's surface. Cagniard teaches that a specific relationship exists between the measured orthogonal components of the earth's magnetic field and the measured orthogonal components of the earth's electric (or "telluric") field, and that spatial variations therein may be used to derive specific information regarding the subterranean structure of the earth. As certain structures are known to be indicative of the presence of minerals, these magnetotelluric measurements are a useful prospecting tool.
The Cagniard method of magnetotelluric exploration usually involves the generation of signals representing the magnetic field and the electric field in two orthogonal directions, usually denoted by H.sub.x, H.sub.y, E.sub.x and E.sub.y respectively. In this type of exploration it is desirable to record the electric field and the magnetic field at spaced locations along a line of exploration; the data thus generated can be combined with data obtained along parallel lines to yield a three-dimensional picture of the subterranean structure. A similar technique is used in seismic exploration; compare U.S. Pat. No. 3,105,568 to Jolly, which is an example of a seismic exploration technique, commonly referred to as "roll along", which obtains multiple coverage of common depth points in seismic exploration.